RELS-EXT update process

[Natkeeran]

With this PR merged, multi_importer offers three ways to update (including a "don't update" option) RELS-EXT. In all three cases, it discards duplicate predicates except for membership predicates . (This maybe because Diego tried to help me out with this issue).

In RELS-EXT, there can be valid duplicate predicates such as below:

<islandora:isManageableByUser>user1</islandora:isManageableByUser>
<islandora:isManageableByUser>user2</islandora:isManageableByUser>
<islandora:isManageableByUser>user3</islandora:isManageableByUser>

Possible solutions:

• For the last option of OLD, don't touch RELS-EXT at all.
• This may be needed for MERGE option as well. (We simply have to treat the multiple inheritXacmlFrom issue as one time regression issue. I've cleaned that up in our local case.)

[Natkeeran]

A cleaner and simpler solution would be to only remove fully duplicate statements (predicate and value).

[DiegoPino]

But would leave issues like sequence number, page number, etc around that would interfere with a lot viewers and SP (imagine moving pages from one book to another). Also would leave other dependencies. The real solution is sadly not easy. Its to (once for all) make a compendium of needed-basic properties. And deal with the not defined there as you suggest here. I will work on this.